Modulation indicator



Dec. 27, 1927. 1,654,090

. A. A. OSWALD MODULATION INDICATOR Filed March 17. 1924 fiyfi 2 i J i25x :8 I F 0 0 IE iF v men/0r:

Ari/2am. wWa/d by MA Patented Dec. 27, 1927.

UNITED STATES PATENT. OFFICE.

ARTHUR A. OSWALD, OF EAST ORANGE, NEW JERSEY, ABSIGNOB 'IO WESTERN ELEC-TRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEWYORK.

MODULATION INDICATOR.

Application filed March 17, 1924. Serial No. 898,716.

This invention relates to the measurement and indication of the degreeof modulation effected in a system for the transmission of signals andmore particularly to a'method of and means for indicating whetheramphfying devices, used to increase the power of the oscillationsrepresenting the signals, are functioning properly.

In the transmission of speech by electric waves, the power of the Wavesvaries through a wide range in accordance with the modulations of thevoice and the energies of the several tones. Certain of the tones, forexample the lower pitched vowel tones of speech, produce oscillations oflarge amplitude and power, while other tonesa're conveyed with a verysmall amount of power. The quality of the transmission and thefaithfulness with which it can be reproduced at the receiving points ofthe system depend upon the ability of the system to transmlt with equaldegrees of purity both the strong and the weak oscillations.

in systems employing amplifiers to mcrease the average level of thetransmission, it is well known that overloading the amphfying devicesproduces distortion of the wave form of the impressed oscillations andin the case of speech serious overloading may render the received speechalmost unintelliible. g @n account of the irregular modulation of speechwaves, modulation indicators that indicate the mean value of the currentare generally insufficient to show what is happening to oscillations oflarge amplitude. The speech of particular indlvlduals or the music ofcertain types of musical instruments may be satisfactorily transmittedat an energy level corresponding to a particular mean currentindication, while the speech of others and other forms of music in whichthe range of modulation is greater may still produce about the same meancurrent indication and yet, on account ot tl1e1r greater range ofintensities, causesenous overloading with resultant distortlon in thereproduction.

in accordance with this invention, a modu lation indicator is provided,by means of which the maximum oscillation amplitudes are measured ratherthan the mean or the efiective values.

In one form of'the invention, a space discharge tube is used inaccordance with the s stem of Patent No. 1,232,919, patented to eisingJuly 10, 1917, to constitute a peak voltmeter. An indicator of this typegives most precise results when operated 1n accordance with a zeromethod, an adjustment of the device being made until the impressedvoltage is neutralized. It is apparent that in using a device for thepurpose of indicating the modulation effected in a transmis- S1011system, it is not feasible to employ a zero method which requirescontinuous adjustment by an operator to secure indica tion. Accordingly,to make the device of the l' leising patent suitable for use as atransmission indicator, a plurality of such voltmeters s used each aduSted to be responsive to impressed voltages of particular amplitudes,the response to these amplitudes being arranged in accordance with aseries of increasing values. 7

In accordance with the foregoing, it is an object of the invention toprovide a modulation indicator responsive without continuous adjustmentto the maximum modulations of the signaling current.

A further object is to provide an indicating system whereby overloadingof a trans mission system may be avoided.

A collateral object is to provide a transmission system having animproved. quality of reproduction.

The use of a plurality of peak voltmeters in accordance with thisinvention provides a visual indication of the modulation in atransmission system. They are readily responsive to the transmittedsignal currents and are able to follow the comparatively rapidfluctuations of energy of low frequency currents corres onding to speechor music.

The amplitude at which distortion due to overloading an amplifyingdevice becomes objectionable, may be determined for any particular typeof amplifier by means of articulation tests. Such tests are capable ofgiving a close correlation between the amplitude and the percentage ofspoken words that are correctly heard. When data of this kind have beenobtained, the application of this invention insures that transmissionmay be accuratel controlled to deliver the greatest amount 0 energyconpredetermined control of the transmission stem in such cases requiresa knowledge 0 the robable maximum amplitudes that may 0 associated witha. given mean current in the particular type of intelligence that isbeing ".r-tm smi t d- "The nature of the present invention and the oeration of its various parts will be more ully understood by reading thefollowing detailed description in conjunction with the drawings, ofwhich Fig. 1 shows the invention applied to a .high power wirelesstelephone transmission system; x

Fig. 2 shows an alternative method of connecting the modulationindicator to the system under control;.

Patent No. 1,434,555, patented to W. H.

Martin November 17, 1922, by means of which a high degree of attenuationis socured at a frequency close to the edge of the frequency bandtransmitted by the filter. The filter is proportioned to pass only oneside band of frequencies and by means of its sharp attenuation close tothe cutoil frequency the suppression of any residual waves of thecarrier frequency Fig. 3 illustrates an alternative method may becompleted. The energy transmitted of connecting the space dischargetubes of the modulation indicator;

Fig. 4 shows another embodiment of the modulation indicator in which twoelement discharge tubes are used as polarized recti-' tures of theoperation of the indicator.

Referring to Fig. 1, 1 and 2 are the wires of a two-way transmissioncircuit which may be connected to 'a distant telephone station. By meansof the bridge coil 3, directional separation of the currents in thetwo-way line is obtained, outgoing currents from the remote stationbeing transmitted through the conductors 6 and 7 to the input of theamplifying and translating apparatus. The two-way line is balanced by anetwork which is indicated conventionally by the rectangle 4. Thewinding 5 of the bridge coil 3 receives the incoming circuits from aseparate branch which is not illustrated as it plays no part in the oeration of the invention. The branch 6-- conducts the outgoing currentsto a potentiometer 8, out of which they pass through the switch 50 tothe speech input circuit of the balanced modulator 9. This modulator maybe of the type described 'in Patent No. 1,343,307, June 15, 1920, to

J. R. Carson. v

A carrier wave'is supplied to the modulator from a high frequencyoscillator 10 which may be of any convenient type. This carrier wave isimpressed upon the input circuits of the modulator in parallel and theamplified carrier component is suppressed by the balanced arrangement ofthe output circuits. The purpose of this first stage of modulation is tosecure complete suppression of the carrier and in order that this may beaccomplished, the frequency of through the filter 11, consisting of asingle side band without any attendant unmodulated carrier, is deliveredto a second modulator 12 which may be of the same type as the modulator9 and is indicated iagrammatically. A second carrier wave having afrequency suitable for radio transmission is supplied to modulator 12 bythe oscillator 13. Modulation in modulator 12 takes place between thesecond carrier wave and the waves of approximately 30,000 C. P. S. ofthe single side band produced by the preceding apparatus. The productstherefore include a pair of side bands separated by approximately 30,000cycles from the frequency of the waves supplied by the oscillator 13.Output energy from the modulator 12 is delivered to a band filter 14which is designed to pass only one of the side bands and hence serves tocompletely surpress the other side band and the carrier frequencycomponents. The single side band transmitted through the filter 14 issupplied to the input circuit of amplifier 15,to which space current issupplied by generator 16 through a choke coil 17 and space current meter18. The amplified side band is delivered to amplifier 19, which may beof the same type as the amplifier 15. The output energy from thisamplifier is transmitted through condensers 41 to a tuned circuit 20which is coupled to antenna 21 by means of coupling coil 22.

A system of this type is typical of wireless telephone systems used forlong distance communication. In view of the large number of stages ofmodulation and amplification used in this system it is necessary that aprecise and reliable means be provided for indicating the degree ofmodulation so that the operation of the system may be contill trolled tosecure themost efiicient transmission.

In order to obtain an indication of the modulations efiected in a systemof this type,

a part of the energy delivered to the antenna is supplied to the circuit25, 26 by means of the coupling transformer 23, 24. The energy deliveredto the circuit25, 26 1s 1mpressed u on the input electrodes of threespace disc arge tubes 27 28 and 29, each of which is provided with afilament H an anode I and a control element J.

The filamentsare connected in series and? are supplied with current fromthe source 30 through a filter 31', the purpose of which is to suppressany fluctuations of the current from the source 30.

the excess voltage and determine the fila ment current strength, butserve also to establish the filaments at certain fixed potentialsrelatively to eachother and to the negative terminal of the supply;

The anodes I of the vacuum tubes are connected through ammeters 40 to acommon lead 39, which in turn is connected to the ositive terminal ofthe source 30. T e impedance control element of tube .27 is conductivelyconnected through a high lrequenc choke coil 37 to the negative terminalo the filament current source and is thus given a negative polarity withrespect to the filament H by an amount determined b the fall ofpotential in resistance 34. ontrol elements of vacuum tubes 28 and 29are similarly connected through high frequency choke coils 37 to thecommon points of resistances 35 and 36 which are connected between therespective filaments and the negative. terminal of the source.

By properly proportioning the resistances 35 and 36 and the resistance34.- to establish the filament potentials, the normal negativepolariaing potentials applied to the control elements or grids of therespective tubes may be maintained at any predeterminedvalues. It isnecessary that the resistances 35, 36 be so large that a minimum amountoi current supplied by the source 30 is diverted from the filaments ofthe tubes 28 and 27.

Conductor 25 is connected to the grids through separate condensers 38which constitute low impedance paths for the high frequency energysupplied through the cir- V cuit 25, 26 and also serve to prevent thepolarizing potentials bung short circuited through the coil 24.Adjustment of the filament current within a lim ted range is indicatorwhich is slightl "polarized also in like manner.

provided for by the resistance 32 which is variable;- 1

When the transmitter is idle, the polarizw tion of the grids J issufiicient to prevent any space current flowing through the dischargedevices and hence it is necessary, when the transmltter is operating,for the wave E. M. F. impressed u on the grids to overcome thispolarization efore any indication is produced.

Fig. 2 shows an arrangement for cou pling the output of the amplifier 19tothe transmission circuit and to the modulation different fromthatshown in Fig. 1. is unit may be substituted for that portion of Fig.1 to the right of vertical line Y-Y and above the horizontal line X--X.A potentiometer 42 is connected across the output circuit of theamplifier 19. The conductor 26 is connected to one terminal of thepotentiometer, and the conductor 25 to the adjustable contact point. Thetuned circuit 20 and the antenna 21 are replaced, in this figure by animpedance 55, which serves to indicate that t e energy may be deliveredto any -t e of load circuit.

ig. 3 illustrates an alternative arrangement which may be used in placeof. that part of Fig. 1 below the line XX. This arrangement differs fromthat of Fig. 1 in thatthe lead 25 through which high freuency energy issupplied to the space the 0 large tubes is connected to their anodes.The filamentsand the grids of the tubes are connected to the supplysource 30 in the same manner as in Fig. 1, and the grids are Condenser43 prevents the circuit 25., 26 from acting as a s ort circuit for thesource supplying plate current to the tubes, and high frequencychokecoil 44 revents the high frequency oscillations rom beingshort-circuited through the direct current source 30.

The arrangement shown in Fig. 4 may also be substituted for that part ofthe apparatus below the line X-X of Fig. 1. Thethree' element tubes 2728 and 29 are replaced by three two-element tubes 425, 46

and 4:7,the filaments of which are supplied with current from source 30through inde-' pendent parallel circuits. Resistances 48 and 49 includedin each filament circuit control the current supplied to the filamentsand also the potentials of the filaments with lid respect to thenegative terminal of the sup of Fig. 1 is as follows: By means of thecoupling coil 23, 24 part of the energy delivered to the antenna isimpressed across the terminals of the. hi h frequency choke coils 37,and consequent y between the grid and filament of the discharge tubes.Normally, no plate current flows in the discharge tubes on account ofthe polarizing potentials impressed upon their grids, and none can flowuntil one or another of the grids has a sufliciently large positivepotential superimposed upon it to offset the blocking efiect of itsstead negative potential. If the amplitude o the impressed oscillationsis sufficiently great, the potential of the positive art of the cyclewill ofi'set the blockin e ect of the negatively polarized grid an causea momentary flow of current in the plate circuit.

The normal potentials impressed upon the grids of the dischar e tubesare graded so that, for example, tu e 27 will be rendered conductive forspace current by the superppsition of a relatively small positive E.

F., space current will flow through tube 28 when a larger positive E. M.F. is impressed upon its grid and space current will flow through tube29 when a still larger positive E. M. F. is sup lied to its grid. Therelationship of these M. F.s is predetermined by the adjustment of theresistances associated with the filament circuits.

If the amplitude of the current delivered from the transmittingapparatus is small, the effect will be such as to produce only anoccasional impulse of space current through the discharge tube 27thereby causing an occasional sharp deflection of the pointer of themeter 40 associated with the tube 27.

As the amplitude of the current is increased,

the tube is more or less continuously conductive and the meterindication will be continuous and will vary approximately in accordancewith the degree of modulation effected. Further increase in the level ofthe transmitted power produces occasional agitations of the meterpointer associated with the second tube 28 due to the amplitudes beingsufliciently great to occasionally overcome the polarization of thegrid. Still further increase in the power level results in continuedagitation of the meter associated with the tube 28 and an occasionalagitation of the meter associated with the tube 29, and so on, eachdevice of the group be.-

coming operative in response to successively increasing amplitudes.

A group of three tubes is shown, but it is evident that as many may beused as desired and the voltage increments at which the succeeding tubesbecome operative may be proportioned in any desired manner.

As the power level of the transmission 7 is increased, the tubesresponsive to the low power values will tend to be overloaded,

between the grid and filament of a thermionic vacuum tube is increasedpositivel the space current increases ro ressive y until a point isreached at w ic all the electrons emitted from the filament arecontinuously drawn off to the plate. After this occurs, furtherincreases in the voltage applied to the space path produce no change inthe space current. The discharge tube is then said to be in a conditionof voltage saturation. The point at which voltage saturation occursdiffers for each value of filament temperature. High temperatures permitthe flow of large space currents, while at lower temperatures the valueof the space current is smaller. Since the filament temperature iscontrolled by the current flowing through it, it follows that for eachparticular current value there is a limiting space current which cannotbe exceeded no matter how great an electromotive force is impressed uponthe electrodes. Hence, by using filament currents of the proper valuesthe point at which voltage saturation occurs in the tubes 27, 28, 29 or45, 46 and 47, may be predetermined so that the amplitude of the spacecurrent flowing through each tube cannot exceed a definite safe value.

In Fig. 5, the operating characteristics are shown in gra hlc form. Theabscissa: represent the amp itudes of the alternating electromotiveforces applied to the grids of the vacuum tubes from the transmissioncircuit. The ordinates represent the intensities of the space currentsas indicated by the meters 40. The curve ODD represents thecharacteristic of the tube 27; curve OEE that of tube 28; and OFF thatof tube 29. The minimum potentials required to neutralize the blockingeffect of the steady grid polarizing potentials are represented by .theabscissae OD, OE and OF res ectively; the saturation current corresponing to the particular filament current is re resented by the ordinate CGand the line (i By adjustment of the resistance 32 the filament currentmay be set at a value tor which the corresponding saturation current issufficient to produce a full scale deflection of the plate currentmeters, but is insufiicient to cause any damage thereto because ofoverloading.

When a group of three tubes is used, as illustrated, it is convenient toadjust the values of the grid polarizing E. M. F.s so

dill! means of switch an that the minimum operating voltage of one ofthe tubes corresponds to the maximum power output from the transmittingsystem consistent with the requirements of the quality of reproduction.When the system is o crating at its full power, the meter associatedwith the tube selected should show no indication or only occasionalsmall deflections, this tube corresponds to the tube 29 of Fig. 1 inaccordance with the fore possible power output of the transmittingapparatus is being supplied to the transmission circuit.

For the purpose of calibration, the irregular modulations of speech inthe transmission system may be replaced by a steady tone resultin in asteady output, the amplitude of which can be measured by means of anammeter of any ordina'r type.

In the system shown in Fig. 1, the speech input circuit of modulator 9may be disconnected from the tele hone line 1, 2 by connected instead toa low frequency oscillator 51. This oscillator may beset to generate apractically pure tone of some frequency, for example 1500 cycles, withinthe range of speech transmission. The voltage output is measured byvoltmeter 52 and may be adjusted by means of potentiometer 54.

The amplitude of the oscillations flowing through t e output circuit ofthe power amplifier 19 may be measured by means of an ammeter53 includedin its output circuit.

In carrying out the calibration, oscillators 10 and 13 are set to givetheir proper frequency and current strengths. The output of the lowfrequency oscillator 51 is ad usted by means of the potentiometer 54until an out at current'is indicated by ammeter 53 in t e output circuitof the power amplifier which corresponds to the known permissiblemaximum space current of the particular amplifiers used. The coupling oftransformer 23, 24 is then adjusted unt l the meter associated with thetube 29 just fails to indicate. The meters associated with the othertubes will then be operated since, as

stated above, they are adapted to operate at successively lower voltagesthe value of which in each case is predetermined by the value of theresistances in the respective filament circuits.

The operation of the modulation indicator of Fig. 3 is similar to thatof the arrangehis ment shown in Fig. 1. The discharge tubes 27, 28 and29 are similarly connected to the direct current source and the spacecurrent is normally suppressed by p0 arizing the grids negatively bydifferent amounts, in the same manner as in the system of Fig. 1.

The voltage derived from the transmission system is applied through theblocking condenser iti to the plate circuits of the vacuum tubes. As iswell known, the control effect upon the space current of a three elementspace discharge device due to an E. M. F. im ressed upon its grids, maybe produced by a proportionally related E. M. F. impressed upon itsanode. In general, this related E. M. F. is larger, its value dependingupon the amplificationconstant of the tube. The arrangement shown inFig. 3 therefore operates in a closely analogous manner to that of Fi 1,the larger electromotive force require for its operation being securedby increasin the coupling of the transformer 23, 24. natively the degreeof polarization of the grids may be correspondingly reduced, making theincreased cou ling between the indicating apparatus an the transmittingsys tem unnecessary.

The two element vacuum discharge tubes 45, 46 and 47 of the modulationindicator shown in Fig. 4 operate as polarized rectifiers. The degree ofolarization of each tube is varied, as in the evices of Figs. 1 and 3,but in this arrangement the olarizationis obtained by making the ano esK negative with respect to the cathodes L. The amount of thepolarization is determined b the resistances 48 and 49, by means ofwhich the cathodes are connected to the current source 30. Theseresistances also serve to limit the current supplied to the filaments ofthe respective tubes, whereby each filament is heated to the propertemperature. The E. M. F. derived from the transmission circuit isapplied through the conductors 25 and 26 to the cathodes and anodes ofthe tubes, the indicators 40 bein included in the individual anodesleads. [his arrange ment operates in a manner similar to that describedabove for the apparatus shown in Figs. 1 to 3, to indicate the de ree ofmodulation efiected by the transmitting system. Each device becomesoperative when the positive half of the impressed oscillations 1ssufficient to overcome the negative olarizing potential applied to itsanode. hen this condition exists, space current is caused to flow and anindication is produced in the meter 40 associated with the tube.

The term modulation is employed in the foregoing description to identifythe variations in the intensity of electric currents corresponding tothe variations of intensity that give the signal or the intelligence tobe transmitted its recognizable characteristics.

lter- ,tudes of the signal waves byr tem described facilitates to someextent the measurement of the side-band amplitudes, but it is notessential that the transmitter be of the carrier suppression type. Whenthe modulation indicator is used in connec tion with a transmittedcarrier system, the initial adjustment and calibration carried out inthe manner already described results in an increase of the polarizingpotentials of the indicator discharge tubes by which the response to theunmodulated carrier is neutralized.

The invention may also be used in connection with low frequencyamplifiers to indi cate the rapidly varying intensities of speech orother signal waves. In such case, it may be regarded as an ener y levelindicator. In all its applications itis to be noted that the separateindicating devices are responsive either directly or indirectly to theamplitudes of the signal waves.

Various arrangements embodying the invention and their respectivemethods of operation have been described and the combinations andfeatures of the invention which are believed to be novel will be setforth in the ap ended claims.

at is claimed is: v

1. The'method of indicating the energy level of a signal transmitted byelectric waves which comprises detecting from the signal bearing waves aplurality of separate uni-directional currents, the number of saidcurrents increasing in accordance with the amplitude of said waves.

2.- The method of indicating the energy level {of a; signal transmittedby electric waves which comprises separately combining the electromotiveforce of the signal bearing waves with a plurality of gradeduni-directional potentials and detecting from each combination a currentwhich is determined in intensity by the difi'erence of the twoelectromotive forces combined.

3. The method of indicating the energy level of a signal transmitted byelectric waves which comprises separately combining the electromotiveforce of the signal bearing waveswith a plurality of gradeduni-directional potentials and detecting from each combination a currentwhich is determined in intensity by the excess of the wave electromotiveforce over the steady potential with which .it is combined.

4:. A modulation indicator comprising a plurality of space dischargedevices, a circuit common to all of said devices and adapted tov receiveand impress thereon signal modulated waves, and a plurality of gradeduni-directional potentials respectively associated with said devices tooppose the space discharge therein whereby each device is made normallynon-conductive and responsive only to signal waves of amplitude greaterthan a predetermined value.

5. An apparatus for lndicating the degree to which a carrier wave ismodulated, comprising a plurality of indicating devices, a spacedischarge device associated with each of said indicating devices, eachspace discharge device responsive to currents having amplitudes ofpredetermined values, each of said predetermined values being differentfrom the others, means for connecting said indicating devices to atransmission circuit to which the modulated wave is supplied wherebysaid Wave is impressed upon said indicating devices to produce anindication corresponding to the amplitude of said modulated wave.

6. In a wave transmission system including a transmission circuit, aplurality of space discharge devices having dischar e electrodesincluding an anode and acatho e, a source of space current connected tosaid anodes, an indicating device amociated with each anode andresponsive to the space current flowing through the device, a polarizingpotential applied to each of said discharge devices whereby the flow ofspace current is normally prevented, each'of said polarizing potentialsbeing different from the others, an input circuit adapted to receive theener of a wave to be supplied to said transmission circuit and toimpress said energy upon the electrodes of said discharge devices,whereby energy in the transmission circuit in excess of predeterminedamplitudes is caused to produce sensible indications.

7. A modulation indicator comprising a plurality of space dischargedevices havlng electrodes including a cathode and an anode, an inputcircuitadapted to receive electrical oscillations and to lmpress themupon said electrodes, a source of energy for heating said cathodes, asource of space current connected to said anodes and cathodes of saiddischarge devices, a space current indicator connected inseries witheach an ode, a source of steady potential, means for applyingpredetermined fractional amounts of sai steady potential to said dischare devices whereby said indicators are made responsive to oscillations insaid input circuit having amplitudes inexcess of predetermined values.

8. A modulation indicator comprising a plurality of space dischargedevices each of said devices including a cathode, an anode and a controlelectrode, an input circuit adapted to receive electrical oscillationsand connected to said cathodes and control electrodes, a source of spacecurrent connected to said anodes and said cathodes, means for indicatingsaid space current associated with each anode, a source of energy forheating said cathodes, and a source of potential connected to saidcontrol electrodes for negatively polarizing them with respect to theircathodes to prevent the flow of space current, the polarizing potentialof each of said control electrodes being different from the others.

9. In combination a modulation indicator in accordance with claim 7, andmeans to limit the energy supplied to the cathodes to produce a tion.

10. A modulation indicator comprising a plurality of space dischargedevices having an anode, a control element and a filamentary cathode, acommon source of supply for the space current of said devices and forenergizing said cathodes, a circuit connectpredetermined voltagesaturaing said cathodes to said source of supply containing resistanceswhereby the energizing currents are limited to predetermined values andwhereby potentials of said cathodes with respect to each other areestablished at values also predetermined, space current indicatorsassociated with said discharge devices, circuits connecting said anodesthrough said space current indicators to said source of supply, an inputcircuit adapted to receive electrical oscillations and to impress thembetween said control electrodes and said cathodes, means for polarizingsaid control electrodes by predetermined amounts so that said dischargedevices are normally inoperative and become operative when the impressedoscillations exceed predetermined amplitudes.

11. In combination a modulation indicator in accordance with claim 10and means for limiting the current supplied to the filamentary cathodesso that the maximum space current of voltage saturation corresponds tothe maximum current capacity of the space current indicating devices.

In Witness whereof, I hereunto subscribe my name this 7th day of MarchA. D., 1924.

ARTHUR A. OSWALD.

